Neighbor has a 2HP 115volt motor that the internal overload has failed on. Was thinking of putting a external overload on it but was trying to figure out what the three wires going to the internal overload do. It's been a week since I looked at this thing. Seems like one was the neutral but am unsure how it works with the other two leads.

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Neighbor has a 2HP 115volt motor that the internal overload has failed on. Was thinking of putting a external overload on it but was trying to figure out what the three wires going to the internal overload do. It's been a week since I looked at this thing. Seems like one was the neutral but am unsure how it works with the other two leads.

You have a common and a NC, NO.

Most only have a NC.

See exhibit 430.9 in handbook

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Neighbor has a 2HP 115volt motor that the internal overload has failed on. Was thinking of putting a external overload on it but was trying to figure out what the three wires going to the internal overload do. It's been a week since I looked at this thing. Seems like one was the NEUTRAL but am unsure how it works with the other two leads.

Most of them operate the same way an external O/L does. Excess current causes a bi-metal contact to open.

If it has failed closed, an external O/L can be installed, and the motor is protected.

If it has failed open, you'll have to get at the O/L. Some are mounted inside the motor, some in the terminal box.

Usually, there are 3 terminals on a dual-voltage O/L. One of them goes to a winding (if it were labeled, it'd be T1), the other two go to the terminal box. They're usually labeled P1 and P2.

Cut the leads from the O/L, get rid of the two that went to the terminal box, and extend the winding lead to the box. Label it T1. You now have the factory version of the motor when it is supplied without O/L protection.

I checked it out today. Looks to me what happens is the common is switched. Three leads goto the overload, common coming in, common going to the motor and a motor lead coming off the windings.

Common coming in is on one side of the over load. Other side goes to motor. Then the motor lead connects to a heater that then connects to the motor side common. When it over heats it opens the common. At least I think the lead is the common. Glanced at it for a minute. Interesting.

The O/L is a two-pole device. Only P1 to the winding (P1) has a heater. P2 is just a switch from P1.

The reason is because current is sensed on T1; it'll be the same regardless of voltage connection.

If the motor is connected for 230, it is a series connection, and the O/L only needs to sense current on P1/T1 and break it if overloaded.

If it's connected for 115, the windings are parallel. Current is the same in both windings, but only one is sensed. Both windings must be de-energized during an O/L. This is why P2 is not used for 230 but is connected to the other winding (T3) on 115.

Last ones I put up were still two wire, but ceiling fans now have a three-wire black box: in series with the light kit and a neutral. They sense current and drive a relay to kill the lights if somebody installs bulbs that pull more amps than should be pulled by (#bulbs)*(max watts). May be a similar thing; would be easy to check.

Thanks Micro for that excellent description. I might have to abduct this air compressor for a few days.

Based off the nameplate data it needs 30 amp overloads, 30 amp wire, and a 60 amp breaker at 120volts. But it comes on a residential style air compressor that wasn't ever wired for that. Wondering if the manufacturer tested it and it doesn't draw that type of load..